This invention relates to a fail-safe railway signal system for detecting an approaching train on a track and more particularly relates to such a system that operates one or more signal relays to indicate such a train approaching a grade crossing, a track section in a block signal system, or the like. The invention will, however, be described below applicable to pick up or to drop a signal relay at a grade crossing, which will effect pick up or dropping of a crossing gate, although it is to be understood that the system output signal may be used to operate other signalling devices, computers and the like.
Pertinent prior art railway signal systems for detecting approaching trains have used various techniques to compensate for train approach distance and speed to achieve minimum down time during which a signal device is activated or a control signal is generated to provide appropriate indications, for example, at a grade crossing or at another track section, whenever a train is approaching, existing in or leaving a crossing area or track section. The control signal may be coupled to a control system including, for example, a computer for automated train control, or may be used to operate a relay, signal lights or the like. Conventionally, the railway signal systems develop an output signal used to pick up or to drop a signal relay that provides electrical isolation between the lower power signal system and the higher power crossing gate operating motor or the like.
Several existing signal systems respond only to a train within the island between the transmitted and the receiver tie points to the track, requiring long islands to provide train detection within a safe time, and the long island increases the difficulty of system installation. Other devices respond to train approach speed, but do not include variable sensitivity features, such devices often requiring plural systems operating at different frequencies for achieving a minimum safe down time of the sigal device. In still other devices train detection at locations outside the island is achieved using a first signal frequency and train detection within the island is achieved using a second signal frequency in order that the system operating on the first signal frequency may recover while the train is in the island, thereby reducing ring-by time required for recovery of the signal relay as soon as the train leaves the island.
One disadvantage with prior art railway signal systems is that without variable sensitivity, a train consisting of only a single car and/or engine may accelerate after approach time prediction to put the engine almost in the crossing before gate actuation, and another disadvantage is relatively long ring-by time, which is a nuisance to motorists. The effectiveness of prior art systems over a wide range of track ballast conditions is limited, and such systems are not automatically self-compensating for operation after an unexpected shunt occurs across the track in a monitored section.